This invention is directed to an apparatus and process for improved sewage treatment utilizing a deep subterranean shaft as an aerobic digester vessel.
With the urbanization and sophistication of societies the knowledge of sewage treatment has steadily grown. Modern man has come to recognize that raw sewage can no longer simply be dumped into his waterways and/or shallow land excavations. Further, even the undeveloped countries of the world are coming to realize that for adequate public health the treatment of sewage can no longer be considered a luxury, but is a necessity.
The majority of municipal sewage treatment is undertaken utilizing systems that are located on a large area of land surface. Normally incorporated in sewage treatment plants are chambers, ponds or the like wherein the sewage is digested by microorganisms. Together with other sewage apparatus such as clarifiers, aerators, etc. large areas of land must be dedicated to the sewage treatment plants. Further, once the land area is used for sewage treatment it normally cannot be subsequently used for other purposes.
In order to reduce the land use and to achieve a greater efficiency in sewage processing, above ground pressurized sewage digestion vessels have been developed. These digestion vessels take advantage of the increased chemical, enzymatic or biological efficiency brought about by increased oxygen solubility achieved using pressurized vessels. Because this pressure must be artificially created via pumps or equivalent devices energy consumption of these systems must be a serious consideration in their development.
Recently advantage has been taken of the hydrostatic pressure which can be created by using a large digester towers having sufficient fluid head to create an increase in pressure. Power consumption for these apparatuses is said to decrease. They are, of necessity, limited in size, i.e., the amount of hydrostatic pressure head that can be created is dictated by engineering considerations of the physical dimensions of their structure.
To circumvent the engineering problems of an above ground reactor vessel which can be used to create a hydrostatic pressure some pilot plant sewage treatment facilities have been constructed which utilize a shaft sunk into the ground as the pressure tank. These have several advantages. The earth itself provides a supporting structure for the pressure vessel and since the shaft has a very small diameter compared to its depth the land use required with these subterranean shafts is markedly reduced.
In using these subterranean shafts a mixture of raw sewage, air, and a microbiological seed solution (recycled sludge) is injected into the bottom of the shaft via a down flow or downcomer section. Circulation is achieved by causing the mixture of components to descend down the downcomer and then rise within the upflow section of the shaft. Thus, as the "liquor" descends the oxygen solubility within the liquor is increased. Because a constant flow is established any particular increment of a liquor within this flow only has, however, a finite process time and amount of oxygen available to it.